I'd say you could do it one of two ways:
You have skills / expertise in the area of the item being appraised, like you have firearm skills with the type of weapon being appraised, or skills in art and you are appraising a painting. Such appraisals might suffer like a -1 in level as the person doing it knows what they're looking at but might not know the market value.
The other method would be using the broker skill. Again, unless the person with the skill also has expertise in the area of what's being appraised, they suffer a -1 in level as it's outside their area of expertise. Here, they know the market but not so much about what they're looking at.
You get the full appraisal if you have both skills with the higher one applied as the modifier. So, if you have broker 1 and art 2 and you are appraising a painting, then the modifier is 2.
If you are a mechanic 1 and checking out a car, then the appraised value modifier would be 0. That is, you know about the vehicle but have little knowledge of the sales market.